The UI may be different but that's not the sole basis of comparing email clients. The functionality of Thunderbird and SeaMonkey in terms of email are very close and the file structures and underlying functional code (other than UI) are very similar..

Is there a Mail-only installer for Windows?I don't need browser or other components.

thx

You can't install only the Mail component, but you can use it without needing to start the browser, after you have configured SeaMonkey to open with Mail & Newsgroups.

Open the browser, select Preferences > Appearance, remove the check mark from Browser under When SeaMonkey starts up, open, then add a check mark Mail & Newsgroups and SeaMonkey will always open with the mail component.

Oh, that reminds me, you can also launch the mail client by using the "-mail" parameter when launching SeaMonkey from the command line. On Windows, there's a SeaMonkey Mail shortcut in the Start Menu that does exactly this.